This invention relates to a process for preparing polyurea and/or polyurea-polyurethane polymers, particularly non-cellular or microcellular elastomeric and structural polyurea-containing polymers.
Polyurethane polymers are well known and used to make a wide variety of elastomeric, structural and foamed articles. These polymers have been conventionally prepared by reacting a polyisocyanate with what is commonly referred to as a "polyol", i.e. a moderate to high equivalent weight polyhydroxyl-containing compound. The polyol provides "soft segments", i.e. flexible segments which provide good impact strength, elongation and other desirable properties to the polymer. Often, a "chain extender", i.e., a lower equivalent weight polyhydroxyl or polyamine containing compound, is also used to provide crosslinking or "hard segments" in the polymer. These chain extenders impart increased modulus and other desirable properties to the polyurethane.
It has been discovered that not only the mere presence of the hard and soft segments but the manner in which these respective segments are distributed throughout the polymer has great effect on its properties. Generally, it has been found that certain properties of the polymer are maximized when the hard and soft segments are well "segregated". Segregation of these domains refers to the presence at a microscopic or submicroscopic level of regions containing a high proportion of hard segments and other regions containing a high proportion of soft segments.
This desired phase segregation can be accomplished in polyurethane polymers by sequentially reacting the polyisocyanate with all or part of the "polyol" prior to the subsequent reaction thereof with the chain extender and the remainder, if any, of the polyol. In this manner a soft segment prepolymer is formed prior to the formation of the final polymer. Such a technique is described, for example, in U.S. Pat. No. 4,297,444.
As an alternative, it is possible to employ a hard segment prepolymer by reacting the polyisocyanate sequentially with all or part of the chain extender and then with the polyol and any remaining chain extender.
In a separate attempt to improve polyurethane polymers, the hydroxyl-terminated materials commonly employed to make the polymer have been replaced to a greater or lesser extent with amine functional compounds. See, for example, U.S. Pat. Nos. 4,444,910 and 4,530,941. The amine functional material gives rise to urea linkages instead of urethane linkages in the polymer. Compared to the urethane linkages, these urea linkages are stronger and more thermally stable.
Although these amine functional materials have the aforementioned advantages, several serious problems are presented by their use. Amines react so rapidly with isocyanate groups that their use is generally restricted to reaction injection molding (RIM) processes. Even in RIM processes, it has been found generally necessary to employ sterically hindered amine chain extenders like diethyltoluenediamine (DETDA) in order to slow the polymerization reaction enough so that a molded article can be made. Further, amines are known to engage in a variety of secondary reactions with isocyanates at room temperature, causing undesirable crosslinking and gel formation in polyurea-containing polymers. It would be desirable to provide a polyurea polymer which is more easily processible.
In addition, it is always beneficial to provide polymers having improved physical properties and processability.
Accordingly, it would be desirable to provide a process for preparing polyurea containing polymers which provides for greater processing flexibility and a product polymer having physical properties which are equivalent to or superior than those exhibited by conventionally prepared polyurea polymers.